As @equalo212 said, you can estimate your Free Testosterone using an online calculator. There's one on the ISSAM website, and another on the Nebido site.
There doesn't seem to be an agreed upon cut-off for what level of free T constitutes hypogonadism, but the line is somewhere between 6.5 and 8 ng/dL, with most specialists using the lower end of that range.
At 5.9 ng/dL, you're in the hypogonadal range. And there's evidence to suggest that low free T causes more symptoms than low TT.
If you want to do a bit more reading here are a paper, and a review of best practices published by Men's Health Boston giving expert guidelines on who should be offered TRT. Note that the contributors are all serious academics in endocrinology and urology.